Sound Advice for Noise Management
We may take this for granted, but we all experience reality through our five senses, not the least of which is our sense of hearing. Aural messages direct us in our daily lives in ways that we barely take notice of, as we discern and disseminate whatever particular sound has value to us, and ascribe the rest to background noise. C.S. Lewis describes incidental or residual sound as a demonic din, straight from hell: Noise—that grand dynamism , the audible expression of all that is exultant, ruthless and virile!
I don’t know if I would go so far as to declare the natural byproduct of human activity as evil, but there are times when the aural assaults do seem to come with a complementary whiff of sulfur. And when the level of infernal noise becomes too overwhelming to be filtered out, we must depend on specially designed environs that mechanically reduce or eliminate the unwanted cacophony.
Sound management in construction is attained through two different approaches to two different issues—sound transmission and sound absorption. Minimization of sound transmission is achieved by blocking sound waves, usually means of an insulative material installed into wall and/or ceiling cavities. Sound absorption is accomplished with acoustical products comprised of deadening materials that are usually exposed to the occupants, and therefore an aesthetic factor comes into play. Acoustical assemblies designed for sound absorption are used extensively in commercial applications (schools, hospitals, hotels and office buildings), while sound transmission treatments are utilized across the board—on commercial, multifamily residential and single-family residential projects.
My Son, the Drummer
Of course there are exceptions to the rule, and some acoustical considerations may be required for single-family homes with special use areas, such as home theaters, music rooms and entertainment centers.
My own home became one of those exceptions around 15 years ago when my son became a drummer. Blake was about 10 when he first caused me to take a long hard look at the world of acoustics. His initial efforts to imitate the drum solo from “Wipeout” by the Surfaris were as impressive as they were intrusive, so I was determined to encourage his enthusiasm and somehow facilitate his newfound talent while restoring some semblance of peace to the household.
After no small amount of brainstorming with other family members, I resolved to create a space for him by partitioning off a 10-by-20–foot portion of a three-car garage. A simple metal stud wall with drywall on both sides and the addition of a door from the house gave him the space he needed, and I felt confident that my project was complete. I was dead wrong.
The Beat Goes On
Unfortunately, the new music room was located directly below my bedroom, and it wasn’t long before the Charlie Watts–inspired fills that my kid was cranking out precluded any chance I might get for an afternoon nap or quiet period for writing. Undaunted, I conceived of an immediate remedy for this revolting development that was certain to be effective.
Having had some experience with installing grid ceilings in the past, I bought some standard grid and acoustical ceiling tile from the local Home Depot and constructed a drop ceiling throughout the new space. Little did I know at that time that sound deadening (acoustics) and reduction of sound transmission are two very different processes, requiring very different products.
Predictably, the rhythmic expressions emanating from my prepubescent Ginger Baker were still penetrating my bedroom floor—lots of rattle, crash and thump, somewhat muted but nowhere near eliminated. At first I blamed this failure on my purchase of low-cost, garden variety ceiling tile with a low noise reduction coefficient rating.
But several of my colleagues who work with acoustical ceilings more than I do explained that acoustical assemblies are primarily designed to mute the noisy reverberations within the source area, while acoustical insulation is better formulated to prevent sound from intruding into other areas. While some types of tile offer some resistance to transmission, upgrading the ceiling tile to a higher performing product would likely render only a very small improvement in noticeable sound transfer. Still, I was determined to resolve the problem to whatever degree possible, so I plunged into the task of selecting a tile replacement that might diminish the “Bo Diddly beat” that shook my bedroom floor on a daily basis.
Numbers Matter
But I had no idea about the vast selection of acoustical ceiling products. In fact, one major acoustical ceiling manufacturer produces more than 138 different mineral fiber ceiling tile types. And this was only one of many manufacturers. These ceiling units generally differ with regard to dimension, by thickness, by configuration (square-edge, beveled tegular and scored) and by performance specification (sound absorption, sound transmission, light reflectance, humidity resistance, fire resistance, impact resistance, wash-ability and recycled content).
Reverberation in the music room was an issue due to the concrete floor, so a good NRC rating for sound absorption was something I needed to consider. NRC is a numerical rating system that represents an average performance of sound absorption at varying levels of high and low pitch noise. NRC ratings range from 0.0 to 1.0, with a higher number indicating a better performance. With 0.5 being a good rating, I guesstimated that my budding Keith Moon’s new lair was in need of a 0.7 tile or better.
But the more critical specification for my tile selection would lie with its Ceiling Attenuation Class rating. CAC is a numerical measure of sound transmission from a source room to an adjacent room (next to or above, as in my case). Ceiling Attenuation Class performance in a tile can be affected by factors of composition, density and thickness, with ratings ranging from 25 to 50. And if 35 would be considered a good CAC tile, I’d be hunting for a 40 or better.
Since the square footage of the music room was not significant, the replacement of the cheap tiles with the upgrade I selected was not a budget-buster. Good thing, because neither was it a resolution to the issue at hand. I noted that the snare drum rolls and paradiddles invading my room had somewhat diminished, but the thump from the bass was still a strong distraction. Clearly, new tile alone was not the answer. Once again my colleagues had to educate me on the benefit of sound insulation.
More Numbers
Batt (or blanket) insulation is the most commonly utilized insulation element, but I was amazed, once again, by the assortment of possible types within that segment alone. Generally speaking, there are two types of batts that perform different functions: those that regulate heat transference and those that manage sound transmission—i.e., thermal and acoustical insulation.
Batts generally come in two different widths, 16 and 24 inches, to accommodate the spacing of wall and ceiling cavities between framing members. Effectiveness of acoustical insulation can be rated by R-value, which stands for resistance, that is, resistance to sound passing through. Obviously, a higher the R-value indicates greater the resistance to transmission, with the level of R-value being determined by type, density, and thickness of the batt.
Concerning type, batts usually consist of one of three materials: fiberglass, mineral wool or cellulose (recycled paper). Density of material is measured by R value per inch of thickness, ranging from 2.9 to 4.3 per inch. Total R performance of a batt is attained by multiplying the per-inch value times thickness. Therefore, a 6-inch thick batt with a density value of 3.5 is rated an R-21.
Armed with all of this newly acquired knowledge, I proceeded to install 200 square feet of R-21 unfaced acoustical batts above my recently installed and upgraded ACT drop ceiling, removing and replacing each tile as I went, taking extra care to eliminate any gaps between batts.
The result was a dramatic reduction to the rumble that had heretofore assaulted my inner sanctum. I had slain the noise monster with a two-punch approach to acoustics. My acoustically astute colleagues speculated that the sound transmission coefficient for my floor/ceiling assembly must be pretty high.
Sound transmission coefficient is a numerical system that rates how well a partition, ceiling or floor blocks airborne sound (the higher the number, the greater the reduction of sound transference through the assembly). For instance, partitions with an STC rating in the 30s allow ordinary speech to be audible from one side to the other, while partitions with an STC rating in the high 50s render loud sounds virtually mute. Between the new drop ceiling I had installed and the existing floor/ceiling combination above, I’d created an acoustically effective assembly, possibly approaching an STC 50.
Wait. There’s More.
But just as I was celebrating my victory over the noise monster in the bedroom, another sound-related problem raised its ugly head. One of the existing walls that bordered the music room was adjacent to the formal living room. It was pointed out to me that this common partition was offering little or no acoustical transfer resistance to our little Neil Peart–wannabe’s rhythmic compositions.
This surprised me because the party wall had previously been an exterior partition (common with the garage), so I had assumed that the requisite thermal insulation would double as an acoustical block in this case. Be that as it may, the thump and rattle from my boy’s kit was clearly penetrating the room, and it could not be ignored. I surmised that I would have to furr the music room side of the wall and add acoustical insulation in the newly created cavity. I wanted to achieve the best STC rating with the least amount of encroachment into the room, so I began doing some research into material density.
Specifically I was looking for material that would provide the greatest R value with the least thickness. I found that doubling thickness will increase sound resistance by up to 400%, while doubling density of material only nets a 20% to 100% resistance increase. I applied both approaches by erecting a 3 5/8–inch metal stud furring wall on the music room side of the offending partition, and filled the cavities with 4-inch thick rigid fiberglass panels that have a density factor of around R-5 per inch of thickness. I selected rigid fiberglass for its proclivity in forming “bass traps,” which seemed to me to be well-suited for my purpose.
I added cross-furring of resilient channel before closing up the cavity with a new layer of 5/8” drywall. Between my added furring and the existing 2-by-6 wall, I believe I achieved another STC 50 assembly, and you could literally hear the proof: From the living room side, one could barely tell that my little drummer boy was wailing on the skins like John Bonham on the other side. I then added a raised floor with carpet and pad to cut down the reverb in the room.
What Drums?
Much to my surprise (and delight), Blake never gave up on drumming, and his long hours of practice were never a nuisance to anyone thanks to my acoustically enhanced assemblies. But around five years after the initial construction of the room, he asked if I could build him a sound booth in the room so he could record some of his work with his group. Of course, I rose to the occasion and partitioned off a closet-sized area, complete with countertops for recording equipment.
When it was done, Blake informed me that the room had to be acoustically tuned to reduce the feedback from what he called noise pollution. I resolved to achieve this with the use of acoustical wall panels. Acoustical wall treatments entail many products, the most common of which is a fabric-wrapped panel with a foam or rigid fiberglass core. Acoustical wall panels are effectively used as bass traps—i.e., low frequency sound management, which promotes better bass clarity.
Bass traps are often used in critical listening environments, like recording studios, control rooms, music rehearsal rooms and home theaters. So I wrapped the entire interior of the recording room with full-height, 2-inch thick panels and created a wainscot of 4-foot high panels in the rest of the music room, for aesthetic as well as acoustic purposes. With these added features, I believe we created something approaching the “acoustically perfect” rehearsal room.
Of course, sound management lends itself most readily to commercial and multifamily environs. But, as stated, single-family homes with special areas may require acoustical enhancement as well. I cite my own experience as a worthy example of this, as I gained a good working knowledge regarding the use and application of acoustical products in a home setting, while facilitating my son’s rhythmic talent.
Ringo Starr should have had a dad like me.
Vince Bailey is an estimator/project manager in the Phoenix area.
